A fuel cell is a device that generates power by electrochemical reaction of hydrogen (fuel) with oxygen. The product of this reaction is water in principle, and therefore, fuel cells are environmentally friendly. Accordingly, fuel cells have been expected to be used for home cogeneration systems, and their development has been promoted.
Generally, Pt or a Pt alloy is used as a catalyst component used as an electro catalyst of a polymer electrolyte fuel cell (Patent Literature 1).
However, when Pt is used as an anode catalyst, the Pt catalyst surface may be poisoned by CO contained in a reformed gas which is obtained by reforming a hydrocarbon-based fuel, such as city gas. The poisoning impairs the activity of the catalyst and increases anode overvoltage. This causes a decrease in power generation efficiency. Therefore, the development of a catalyst component in which this poisoning by CO is reduced has been required.
In addition, when Pt is used as a cathode catalyst, the cathode overvoltage is large, and the cathode overvoltage causes a decrease in power generation efficiency. Therefore, the development of a catalyst component in which this overvoltage is reduced has been required.
Further, Pt is a metal that is very expensive and is also rare in resources, and therefore, it is feared that Pt will be restricted in terms of cost and the depletion of resources when fuel cells will become widespread in the future. From this viewpoint, the development of a non-Pt-based catalyst component has been required.
It is reported that a pyrochlore type oxide catalyst exhibits high activity as a metal oxide-based non-Pt electro catalyst. It is known that a pyrochlore type oxide catalyst exhibits high hydrogen oxidation activity and CO resistance as an anode catalyst for a fuel cell (Non-Patent Literature 1).
It is known that a pyrochlore type oxide catalyst also exhibits high performance as a cathode for a fuel cell (Non-Patent Literatures 2 and 3).
An electro catalyst for a fuel cell is required to have a large specific surface area for both of the cathode and the anode to exhibit high performance. Generally, the specific surface area of a catalyst may often be increased by way of using carbon black or the like having a large specific surface area and good electronic conductivity as the support.
However, in conventional pyrochlore type oxide catalysts, prolonged heating at a temperature close to 100° C., followed by a calcination step at a temperature approximately 300° C. are necessary during the preparation of the catalyst, and therefore, a decrease in specific surface area associated with an increase in particle diameter has been unavoidable in these processes.
Further, calcination in an oxidizing atmosphere or at a high temperature during catalyst preparation is necessary. Therefore, it has been difficult to use a method, such as allowing a substance having low heat resistance or low oxidation resistance to coexist during catalyst preparation. For example, it has been impossible to use a method of allowing a support having a high specific surface area, such as carbon, to coexist during synthesis.